Each year some birds that breed far to the north choose to overwinter here in the southern mountains. Winter residents that come readily to mind are ruby-crowned kinglets, yellow-rumped warblers, purple finches, and fox, white-throated, and swamp sparrows. Hermit thrushes, pine siskins, and red crossbills are often listed as winter residents, but this is not totally accurate since these species do breed frequently in the southern mountains.
One of the most spectacular of our winter residents is an unreliable visitor. Some years nary an evening grosbeak is to be seen. (That was Elizabeth’s and my experience in 1998-99.) Other years they seem to be everywhere. Ornithologists designate peak years as “light years” and massive southward movements as “irruptions.” Other bird species that periodically irrupt southward in great numbers include white-winged crossbills and northern goshawks. John C. Kricher describes the phenomenon in A Field Guide to Eastern Forests of North America (1988):
“These dramatic mass movements ... are unusual both because they involve large numbers of birds and because, unlike migration, they are not generally predictable .... There is no local indication that an irruption will occur.... The appearance of irruptive species is called a ‘flight year’ ... thought to be caused by periodic unpredictable food shortages in the breeding ranges of these species. Seed-eating species may irrupt in years following the cessation of masting. Many young are produced when seeds abound during masting, producing an over population. When the seed crops drop precipitously, seed-dependent species ... are forced southward. Not all individuals of the irruptive species leave the nesting areas, however. Irruptive flocks tend to be comprised predominantly of young birds. Of adults, females seem to outnumber males, although data are not well established on this point.”